The present invention relates to a process for producing unburned basic refractories.
Various unburned basic refractories of magnesite type, dolomite type and the like have heretofore been applied to a converter, a torpede car and other furnaces. With the development of a steel making process using an L. D. converter, a steel of higher quality has been obtainable at a low cost. However, the refractory used in the converter is corroded and/or eroded by molten steel and slag, and as a result the durability of the refractory is decreased. Therefore, it is very important that the durability of the refractory is allowed to increase.
It has been known heretofore that carbonaceous substances show a strong resistance against wetting and penetration of molten steel and slag and therefore various binders with a high fixed carbon content have been developed. A binder for refractories must satisfy various conditions wherein not only the fixed carbon content is high, but also no atmosphere contamination occurs during its use. Also, the price should be inexpensive the boiling range should be larger and the strength of the green body as a binder should be sufficiently strong.
Typically, a coal tar pitch has been hitherto used due to the fact that the above-mentioned conditions are substantially satisfied. However, coal tar pitch contains a large amount of nitrogen- and oxygen-containing compounds which are harmful to human beings and for this reason many serious sanitary problems exist for workers at the time of producing or using the refractories. Therefore, it is desirable to obtain a substitute for the coal tar pitch which does not possess the above-mentioned disadvantages.
Under these circumstances, various attempts for the investigation of a substitute have been carried out. For example, there has been proposed for use resins which have a comparatively high fixed carbon content, such as liquid phenol resins, liquid furan resins and the like. However, these resins contain a large amount of unreacted components, for example the phenol resin contains phenol, formaldehyde and others and the furan resin contains furfuryl alcohol, furfural and others. Therefore, in these resins there is an offensive odor which is extreme, and the atmosphere contamination is excessive and, accordingly, care in handling these resins must be taken. Also, the moistureproof property of these resins is poor. Particularly, the dolomite refractories absorb moisture which is present in the air during the storaging of refractories and as a result slaking spots and cracks are generated which disintegrate the refractories. Due to these facts, it is desirable not to use these resins.
When the coal tar pitch is applied, the pitch is usually diluted with a solvent to facilitate workability in producing the refractories and accordingly it is necessary to use a creosote oil or an anthracene oil in view of the solubility of coal tar pitch. As a result, there is the disadvantage that such toxic solvents must be employed.
The object of the present invention is to provide a process for producing unburned basic refractories wherein the slag resistance and slaking resistance of unburned basic refractories are improved by using a new type of binder.
As a result of various investigations of thermoplastic resinous materials which are effective as binders for unburned basic refractories, according to the present invention, it has been found that by using a thermoplastic resinous material which is obtained by removing a light fraction having a boiling point of less than 200.degree. C under a reduced pressure of 100 mmHg and having a benzene-insoluble fraction content of less than 10% by weight from a heavy oil obtained by the steam cracking of petroleum, or from the polymerization product of said heavy oil in the presence or absence of a catalyst or from a mixture of said heavy oil and said polymerization product, the above-mentioned disadvantages are eliminated and unburned basic refractories having the same or greater durability than that of products which are obtained by using the prior art binders of coal tar pitch or resins are obtained.
Therefore, the present invention is directed to a process for producing unburned basic refractories in which a binder is added and kneaded with a basic refractory material and the kneaded mixture is pressformed, characterized in that the binder is a thermoplastic resinous material which is obtained by removing a light fraction having a boiling point of less than 200.degree. C under a reduced pressure of 100 mmHg and having a benzene-insoluble fraction content of less than 10% by weight from at least one member selected from the group consisting of a heavy oil obtained by the steam cracking of petroleum and a polymerization product of said heavy oil.
The thermoplastic resinous material according to the present invention has characteristic properties wherein the fixed carbon content is high and the benzene-insoluble fraction content is very low.
It has been considered that in the prior art binders the amount of .beta.-resin in which a quinoline-insoluble fraction is removed from a benzene-insoluble fraction must be increased, and that the carbonization percentage of the binder and the physical properties of the carbon product become excellent with an increase in the amount of the .beta.-resin.
On the contrary, however, the thermoplastic resinous material according to the present invention contains a very low benzene-insoluble fraction and substantially none of the quinoline-insoluble fraction, and despite these facts the fixed carbon content is very high. Therefore, the thermoplastic resinous material, according to the present invention, shows good characteristics equal to or superior to the coal tar pitch.
In addition, the properties of the binders, especially the melt viscosity must be controlled properly according to the performance of the kneader. In this case, various kinds of solvents can be applied to the thermoplastic resinous material according to the present invention which contains a very low content of the benzene-insoluble fraction. Petroleum solvents, such as for example, 70 pale oil, lube oil fractions such as SAE 10, 20 and 30, bright stock, and heavy fuel oil can be preferably applied to the present invention, in spite of the fact that they are not able to be used in the case of coal tar pitch. Thus, the above-mentioned disadvantages can be eliminated.
The solubilities of the thermoplastic resinous material according to the present invention and coal tar pitch into B heavy fuel oil which is one of petroleum solvents, was investigated. The results are shown in Table 1.
Table 1 __________________________________________________________________________ I Thermoplastic res- II inous material Thermoplastic obtained as residue resinous material by steam cracking obtained by heat- of gas oil. treating at 400.degree. C Fraction with boil- the residue obtain- ing point of more ed by steam crack- III Sample than 250.degree. C/100mmHg ing of gas oil Hard tar pitch __________________________________________________________________________ Softening 100 110 82 point (.degree. C) Conradson carbon 40 49 53 (% by wt.) Benzene- insoluble 3 16 35 fraction (% by wt.) Quinoline insoluble 0 0.2 7 fraction (% by wt.) Completely and Insoluble particles Separation of two uniformly dis- are separated to layers occurs State of* solved give a suspension and the two compo- mixture and are precipi- nents are scarcely tated on standing dissolved in each other __________________________________________________________________________ *Sample is mixed with B heavy fuel oil in the ratio 1:1 (by wt.) and the resultant mixture is heated at 150.degree. C.?
From these results it is clear that the thermoplastic resinous materials which are used according to the present invention have a superior function and effect relative to the prior tar pitch product with respect to solvent compatibility. Therefore, a complete impregnation can be realized.
The thermoplastic resinous material which may be used according to the present invention is one which contains less than 10% by weight of benzene-insoluble fraction and which is obtained by removing a light fraction having a boiling point of less than 200.degree. C under a reduced pressure of 100 mmHg and having benzene-insoluble fraction content of less than 10% by weight from a heavy oil obtained by the steam cracking of petroleum, for example naphtha, kerosene, gas oil or a mixture of these petroleum hydrocarbons at about 700.degree. to 1,000.degree. C or obtained from the polymerization product of said heavy oil in the presence or absence of a catalyst or from the mixture of said heavy oil and said polymerization product.
Without removing light fractions with boiling points of less than 200.degree. C, under a reduced pressure of 100 mmHg, inflammable gases will be liberated during the time of producing unburned basic refractories from said thermoplastic resinous material which may cause a fire. Furthermore, light fractions vaporize in a very short time while said kneaded clinker is press formed and burned at 300.degree. to 400.degree. C which results in cracks in the final products and a decrease in durability.
The polymerization of said heavy oil can be carried out in the absence of a catalyst, but it is preferable to use a catalyst when said heavy oil is polymerized. As examples of the catalyst, there can be mentioned the salts of iron, tin, aluminium or zinc, such as for example the chloride and nitrate parts of said metals, and a free radical polymerization catalyst, such as for example benzoylperoxide and azobisisobutyronitrile. An oxidative polymerization using oxygen or oxygen-containing gases or oxygen-liberating compounds can be also adopted. The reaction temperature is suitably selected from room temperature to 350.degree. C depending upon the particular catalyst utilized. In case an oxidative polymerization is conducted a temperature of 200.degree. to 300.degree. C is preferably selected. On the other hand, the reaction pressure is preferable either at normal pressure or a pressure higher than normal pressure. The reaction time may be freely selected according to the desired properties of the polymerization product.
The thermoplastic resinous material which is used in the present invention must satisfy the above-mentioned physical properties as a binder. For this reason it is preferable to use a thermoplastic resinous material which has a softening point of 70.degree. to 120.degree. C, a melt viscosity at 250.degree. C of less than 1,000 centipoise, a fixed carbon content (Conradson carbon) of 30 to 60% by weight and also a benzene-insoluble fraction content of less than 10% by weight, preferably less than 5% by weight. If the softening point is too low, the fixed carbon content decreases and the strength of the green body is poor and consequently it can not withstand use. On the other hand, if the softening point is too high, the thermoplastic resinous material must be diluted with a large amount of suitable solvent (cutback) and as a result cracks and a decrease in durability of the product occur.
The thermoplastic resinous material is preferable add in an amount of 3 to 10% by weight based on normal raw refractory materials. In this case, when an amount is less than 3% by weight is added, the porosity of the refractories increases, the strength decreases, a sufficient fixed carbon content can not be obtained and thus there is no realization of the desired slag resistance. On the other hand, an amount more than 10% by weight is added, the press-forming efficiency of the body decreases because the body to be formed is adhered to a forming press dies at the time of pressing.
As examples of refractory materials which may be used according to the present invention, there are mentioned the dolomite type, a dolomite clinker, a synthetic magnesite-dolomite clinker; the magnesia types, a concretely sea water magnesia clinker, an electrofused magnesia clinker and natural magnesite. According to the necessary purpose, anyone of these types of refractory materials or a dolomite-magnesia mixture type can be used. Further, a minor amount of carbon or graphite can be also added to the refractory material in order to increase a carbon content in the refractories. It is preferable to adopt a refractory material having a particle size of less than about 5 mm in order to suitably regulate the porosity of final product.
The refractory material and the binder are kneaded in a normal heat-kneading machine to produce a body for press-forming. The kneading temperature is defined depending upon the softening point and the melt viscosity, and in particular it is preferably a temperature at which thermal decomposition of the binder does not occur, for example less than 250.degree. C. The melt viscosity depends upon the ability of the kneading machine, and is preferably less than 1,000 centipoise and it is also possible to dilute the body to be kneaded with a lubricating oil fraction or a heavy oil in order to regulate the viscosity.